1. Field of the Invention
This invention relates to a video camera, and more particularly, is directed to a video camera which has a shading correction circuit.
2. Description of the Prior Art
A conventional color video camera with shading correction circuits will be described with reference to FIG. 6, in which an optical system 1 of the camera is assumed to include mechanisms for zoom and iris adjustment (not separately shown). Light L from optical system 1 is incident on a red charge-coupled device (CCD) 2, a green CCD 3 and a blue CCD 4. Each of the CCDs is of the in-line transfer type in which vertically extending image-sensing stripes and charge transfer stripes are alternately arranged and are adjacent to each other. Vertically extending light-shielding stripes are formed on the charge transfer stripes.
The red, green and blue image signals produced by CCDs 2, 3 and 4 are supplied through preamplifiers 5, 6 and 7 to shading correcting circuits 8, 9 and 10, respectively. Respective corrected video signals are output from shading correction circuits 8, 9 and 10 through video amplifiers 12, 13 and 14 to output terminals 15, 16 and 17.
Shading correction circuits 8, 9 and 10 each have two cascaded integration circuits, which receive horizontal and vertical synchronizing signals from a horizontal and vertical synchronizing signal generator 11. Shading correction circuits 8, 9 and 10 produce respective saw-tooth wave signals and parabolic wave signals for the horizontal and vertical periods. Also, each of the shading correction circuits has respective multipliers for receiving the horizontal and vertical period saw-tooth wave signals and parabolic wave signals and multiplying the signals by the color image signal received from the respective preamplifier 5, 6 or 7.
As is well known, shading correction is performed because the intensity of the light provided through the camera's optical system decreases as the distance from the center of the image increases. The parabolic signals referred to above are used for the purpose of compensating for such decrease in light intensity.
For example, FIG. 3 shows light intensity curves at various aperture settings for a wide-angle lens setting. Similarly, FIGS. 4 and 5 show light intensity curves for standard and telephoto lens settings, respectively. In FIGS. 3-5, the horizontal axis shows distance from the center of the image while the vertical axis shows how the light intensity varies with distance from image center relative to the light intensity present at the center of the image. In FIG. 3, curve P10 shows the decrease in light intensity for an F5.6 aperture setting, while curves P11 and P12 are the light intensity curves for F2.8 and F1.7 aperture settings, respectively. It will be seen that curve P11 shows a more rapid decrease than curve P10, and curve P12 decreases still more rapidly than curve P11.
Similarly, as shown in FIG. 4 for the standard lens setting, light intensity curves P20, P21 and P22 are respectively for the F5.6, F2.8 and F1.7 aperture settings. Again, the curves are listed in the order of more rapid decrease in light intensity.
Also, curves P30, P31 and P32 of FIG. 5 are for respective aperture settings F5.6, F2.8 and F1.7 for a telephoto lens setting, and, as before, the curves are listed in order of more rapidly decreasing light intensity.
As can be seen from FIGS. 3-5, the decrease in light intensity from the center toward the periphery of the image, also known as shading, varies with both the lens and aperture settings of the optical system. It should also be noted that all of the light intensity curves shown in FIGS. 3-5 diverge, to a greater or lesser extent, from being parabolic. Accordingly, using precisely parabolic waves as correction signals results in errors in shading correction. The errors in shading correction are particularly significant in the case of the reduction of the light intensity ratio from the periphery to the center of the image caused by aperture eclipse and in the case of f-drop (i.e., reduction of the f-number) at telephoto lens settings. Although it has been proposed to adjust the levels of the parabolic signals before using the signals for shading correction, errors in shading correction remain significant even when such adjustment is used.